The present application relates to digital communications and more particularly to systems and methods for estimating the response of a channel between two nodes of a communication network.
Orthogonal frequency division multiplexing (OFDM) systems offer significant advantages in many real world communication systems, particularly in environments where multipath effects impair performance. OFDM divides the available spectrum within a channel into narrow subchannels. In a given so-called “burst,” each subchannel transmits one data symbol. Each subchannel therefore operates at a very low data rate compared to the channel as a whole. To achieve transmission in orthogonal subchannels, a burst of frequency domain symbols are converted to the time domain by an IFFT procedure. To assure that orthogonality is maintained in dispersive channels, a cyclic prefix is added to the resulting time domain sequence. The cyclic prefix is a duplicate of the last portion of the time domain sequence that is appended to the beginning. To assure orthogonality, the cyclic prefix should be at least as long as the duration of the impulse response of the channel.
To maximize the performance of an OFDM system, it is desirable that the response of the channel be known at the receiver end of the link. To provide the receiver with knowledge of the channel response, the transmitter typically includes training symbols as part of the frequency domain burst. The training symbols have known values when transmitted and their values as received may be used in determining the channel response.
One technique for estimating channel response based on received training symbol values is disclosed in WO 98/09385, the contents of which are herein incorporated by reference. A modification of this channel estimation technique that takes into account channel components having known response is disclosed in U.S. application Ser. No. 09/234,929, the contents of which are herein incorporated by reference.
Typically, the training symbols are interspersed among the data symbols in the frequency domain burst. A limited number of such training symbols are sufficient to characterize the overall channel response. A problem arises, however, if a narrow band interferer signal corrupts reception of a particular training symbol. The value of that training symbol as received will then reflect not only the channel response but also the interference. This will cause the channel estimation procedure to misestimate the channel response at the training symbol position and at surrounding data symbol positions within the frequency domain.
What is needed is a technique that will provide improved estimation of channel response in an OFDM system in the presence of interference that corrupts transmission of training information.